This section of the written disclosure provides background information related to conventional power transfer systems of the type used in four-wheel drive motor vehicles and is not necessarily prior art to the inventive concepts disclosed and claimed in this application.
Power transfer systems are utilized in four-wheel drive (4WD) and all-wheel drive (AWD) motor vehicles for selectively directing power (i.e. drive torque) from the powertrain to primary and secondary drivelines. In 4WD vehicles, the power transfer system is usually configured to include a power splitting device, commonly referred to as a transfer case, arranged to normally transmit drive torque to the primary/rear driveline and selectively transmit a portion of the total drive torque to the secondary/front driveline. Typically, such transfer cases include a rear output shaft interconnecting the powertrain to the rear driveline, a front output shaft interconnected to the front driveline, a transfer assembly drivingly coupled to the front driveline, a mode clutch operably disposed between the rear output shaft and the transfer assembly, and a mode shift mechanism operable to shift the mode clutch between a disengaged condition and an engaged condition. With the mode clutch operating in its disengaged condition, the transfer assembly is disconnected from the rear output shaft so as to establish a two-wheel drive mode. In contrast, the mode clutch is operable in its engaged condition to drivingly connect the transfer assembly to the rear output shaft so as to establish a four-wheel drive mode.
In “part-time” power transfer systems, the transfer case is equipped with a dog-type positive-locking mode clutch and a mechanical mode shift mechanism that can be actuated manually (i.e. via an operator-actuated shift lever) or electrically (i.e. via an electric motor). Typically, such dog-type mode clutches include a mode sleeve splined for rotation with the rear output shaft and which is axially moveable thereon via the mode shift mechanism between disengaged and engaged positions with respect to a clutch component coupled to the transfer assembly for respectively shifting between the two-wheel drive mode and a “locked” four-wheel drive mode.
It is also known to use “on demand” power transfer systems for automatically distributing drive torque from the powertrain to the front and rear drivelines, without any input or action on the part of the vehicle operator, when a low traction condition is detected. Modernly, the on-demand feature is incorporated into transfer cases by replacing the dog-type mode clutch and mechanical mode shift mechanism with a multi-plate friction clutch assembly and a power-operated clutch actuator that are interactively associated with an electric control system and a sensor arrangement. During normal road and driving conditions, the friction clutch assembly is maintained in a released condition such that virtually all drive torque is transmitted to the rear wheels via the rear driveline and the two-wheel drive mode is established. However, when the sensors anticipate or detect a low traction condition, the power-operated clutch actuator is actuated to engage the friction clutch assembly for transmitting a portion of the total drive torque to the front wheels via the front driveline, thereby establishing an “on-demand” four-wheel drive mode. Examples of such on-demand or “active” transfer cases are disclosed in U.S. Pat. Nos. 8,091,451; 8,316,738; and 8,678,158.
To accommodate differing road surfaces and conditions, many transfer cases are also equipped with a two-speed range unit, a range clutch and a range shift mechanism. The two-speed range unit typically includes an input shaft directly driven by the powertrain, a planetary gearset having an input member driven by the input shaft and an output member driven at a reduced speed relative to the input member. The range clutch is usually a dog-type positive-locking range collar splined for rotation with the rear output shaft and axially moveable thereon between a first or high-range position coupled to the input member and a second or low-range position coupled to the output member.
In some part-time two-speed transfer cases, an “integrated” shift mechanism combines the functions of the mode shift mechanism and the range shift mechanism to provide coordinated actuation of the mode and range clutches so as to establish locked four-wheel high-range and low-range drive modes. For example, U.S. Pat. No. 5,159,847 discloses a shift system employing a rotatable sector plate configured to control coordinated movement of the dog-type mode clutch and the dog-type range clutch. As an alternative, U.S. Pat. No. 7,240,577 discloses a motor-driven camshaft operable to rotate a shift cam which, in turn, is configured to control coordinated movement of the dog-type range and mode clutches. As an alternative to such part-time two-speed transfer cases, U.S. Pat. No. 8,037,984 discloses an on-demand two-speed transfer case configured to employ a motor-driven shaft to rotate a range cam for actuating the dog-type range clutch and operate a ballramp unit for actuating the multi-plate friction-type mode clutch.
From the prior art transfer cases noted above, it is known to use a range cam to convert rotary movement of an actuator shaft into sliding axial movement of the range collar on the rear output shaft. Typically, a range fork interconnects the range collar to the range cam. Particularly, a bifurcated fork segment of the range fork is retained in an annular groove formed in the range collar while a follower segment of the range fork is retained in a guide slot formed in the range cam. The guide slot is configured to include a high-range dwell segment and a low-range dwell segment interconnected by a range shift segment. The high-range dwell segment is configured to locate and retain the range collar in its high-range position (i.e. coupled to the input member of the planetary gearset) so as to permit operation of the transfer case in the two-wheel high-range and four-wheel high-range drive modes. Similarly, the low-range dwell segment is configured to locate and retain the range collar in its low-range position (i.e. coupled to the output member of the planetary gearset) so as to facilitate operation of the transfer case in a four-wheel low-range drive mode. The range shift segment of the guide slot in the range cam is configured to include a linear (i.e. single rate) camming profile operable to generate the axially-directed shift force required for moving the range collar (via the range fork) between its high-range and low-range positions to facilitate the range shift function. In some range shift systems, a spring-loaded arrangement is provided between the range cam and the range collar to permit completion of the range shift operation in the event of a tooth block situation. The magnitude of this shift force required to move the range collar and complete the requested range shift is related to the torque generated by the electric motor (and its gear reduction) to rotate the range cam. However, modern automatic transmissions now require the range shift mechanism in the two-speed transfer case to generate larger shift forces which, in turn, have necessarily resulted in increased costs associated with use of larger electric motors and more robust shift system components.
Thus, a recognized need exists to develop alternative power-operated range shift systems for use in two-speed transfer cases that are configured to generate higher shift forces without corresponding increases in packaging size, weight of components and overall product cost.